The chart-recording of a variable provides a preserved visible account of how the value of the variable alters during the running period of the recorder. If the recording of the variable to a high degree of accuracy and/or for a large range of values of the variable is required it may be necessary to allow a large range of pen movement on the chart.
There may in many cases be a second variable the values of which may be of use in conjunction with the values of the first variable or for interpreting the significance of the values of the first value. It is known to record the values of such a second value simultaneously with the first variable by means of a second pen movable over the same chart, but if this expedient is adopted it may be necessary to confine the two pen movements to respective zones of the chart in order to preclude confusion between the two traces or between the relevant graduations.
When considering this difficulty we perceived that in many cases it may not be essential to record the second varaible continuously and an object of the invention is to provide a chart-recording system with chart-recording of a first variable with a pen in for example any normal way coupled with recording of the second variable on the same chart repetitively, at intervals or at selected moments or occasions in a way which enables the records of the two variables to be clearly distinguished and which does not demand any diminution in the pen movement range allotted for the recording of the first variable.